Flowering time in plants varies greatly depending on temperature, the duration of daylight (photoperiod), or both. Generally, according to the relationship between the photoperiod and the flowering time, plants are largely divided into three classes; i.e., long daylight plant which flowers under long daylight, short daylight plant which flowers under short daylight, or day-neutral plant which flowers independent of daylight Such flowering characteristic is believed to be under the basic control of several genes (Yaron Y. Levy and Caroline Dean (1998) The Plant Cell, 10: 1973–1989).
Many studies have been carried out to examine various kinds of mutants, genes, or a pathway controlling the flowering, which affect the flowering time in plants. As a result, it is found that there are three pathways controlling the flowering in Arabidopsis thaliana, of which flowering is stimulated under long daylight The first pathway is an autonomous pathway, in which the flowering is controlled with no connection with the duration of daylight For this pathway, genes of LD, PGM1, FY, FCA, FPA, FLD, etc. are found to be related thereto (Chentao Lin, Plant Physiology, 123: 39–50, 2000). The second pathway is a photoperiodic pathway, in which the flowering in plants is controlled by sensing the duration of daylight Genes of ELF3, CAM1, GI, CO, FWA, FT, FE, etc. are known to play an important role in this pathway (Yaron Y. Levy and Caroline Dean (1998), The Plant Cell, 10: 1973–1989). The third pathway is a vernalization pathway, in which the flowering is controlled by temperature. In this pathway, the flowering of plants is simulated by their exposure to low temperature for a certain period of time. Relating genes of VRN1, VRN2, FRI, FLC, etc. were isolated.
Meanwhile, flowering time is important in crops. For green leaf vegetables such as lettuce, spinach, and dropwart, etc., their leaves quickly become aged after the flowering, and therefore their market value is significantly lowered. Grain crops are divided into three varieties depending on their growth time from sowing to flowering, i.e., early variety, medium variety, and late variety. Early variety yields relatively a low amount of harvest due to its short growth time, but it is advantageous in that it can be harvested early or on the market early. For these and other reasons, flowering time has been the important subject of classic breeding in agriculture.
The breeding method used in classic breeding is typically a cross-breeding method. However, according to this method, it is impossible to introduce specifically one or two genes into a desired crop. As such, a group of unnecessary genes has to be removed in order to have only a character of the desired gene after the breeding, and thus to have the character fixed. To do so, it takes usually a long period of time of 5 to 20 years and lots of efforts. Further, the resulting crop variety which is fixed according to the above method still can display recessive character or sensitivity to pathogenes that have not been considered during the process of breeding and therefore causing a trouble after it is made available to the public. Since breeding to control the flowering time is also based on the conventional cross-breeding method, problems are present For example, instability of breed variety and excessive amounts of time and efforts required therefor, etc. Recently, however, it is possible to isolate genes related to the control of flowering time and to utilize them in breeding with appropriate manipulation of the genes, all thanks to the development in genetic engineering technology. In results, it is expected to have new breed varieties of which flowering time is either artificially controlled or can be possibly controlled (Ove Nilson and Detlef Weigle, Current Opinion in Biotechnology, 8: 195–199, 1997). In this connection, studies have been carried out to isolate a gene inducing mutation from a mutant which expresses a phenotype of eloped flowering time. For example, flowering-controlling genes such as OsMADS5˜8, MdMADS3 and MdMADS4 are disclosed in the publication of Korean patent application No. 1999-0030639, and GIGANTEA gene which controls flowering time and biological clock in Arabidopsis thaliana is disclosed in the publication of Korean patent application No. 2001-0029127. However, taken together the study results up to the present, it is believed that the control of flowering time in plants involves quite complicated pathways and various genes (Alon Samach and George, Coupland BioEssays, 22: 38–47, 2000). Therefore, studies on new genes controlling the flowering time in plants and functional studies therefor are required.